public class LinkedList<E> {
    private class Node {
        public E e;
        public Node next;

        public Node(E e, Node next) {
            this.e = e;
            this.next = next;
        }

        public Node(E e) {
            this(e, null);
        }

        public Node() {
            this(null, null);
        }

        @Override
        public String toString() {
            return e.toString();
        }
    }

    private Node dummyHead;
    private int size;

    public LinkedList() {
        dummyHead = new Node(null, null);
        size = 0;
    }

    public int getSize() {
        return size;
    }

    public boolean isEmpty() {
        return size == 0;
    }

    /*在指定位置（>= 0）添加新元素 时间复杂度 O（n/2）= O(n)*/
    public void add(int index, E e) {
        if (index < 0 || index > size)
            throw new IllegalArgumentException("Illegal index");

        Node prev = dummyHead;
        for (int i = 0; i < index; i++) {
            prev = prev.next;
        }
//            Node node = new Node(e);
//            node.next = prev.next;
//            prev.next = node;
        prev.next = new Node(e, prev.next);
        size++;
    }

    /*在链表头添加新的元素 时间复杂度O（1）*/
    public void addFirst(E e) {
//        Node node = new Node(e);
//        node.next = head;
//        head = node;

//        head = new Node(e, head);
//        size++;
        add(0, e);
    }

    /*在链表末尾添加新元素 时间复杂度 O（n）*/
    public void addLast(E e) {
        add(size, e);
    }

    public E get(int index) {
        if (index < 0 || index >= size)
            throw new IllegalArgumentException("Illegal index");

        Node cur = dummyHead.next;
        for (int i = 0; i < index; i++) {
            cur = cur.next;
        }
        return cur.e;
    }

    /*获取链表的第一个元素*/
    public E getFirst() {
        return get(0);
    }

    public E getLast() {
        return get(size - 1);
    }

    /*查找链表中是否有元素 时间复杂度 O（n）*/
    public boolean contains(E e) {
        Node cur = dummyHead.next;
        while (cur != null) {
            if (cur.e.equals(e))
                return true;
            cur = cur.next;
        }
        return false;
    }

    /*删除链表中指定位置的元素*/
    public E remove(int index) {
        if (index < 0 || index >= size)
            throw new IllegalArgumentException("Illegal index");
        Node prev = dummyHead;
        for (int i = 0; i < index; i++)
            prev = prev.next;
        Node delNode = prev.next;
        prev.next = delNode.next;
        delNode.next = null;
        size--;

        return delNode.e;
    }

    public E removeFirst() {
        return remove(0);
    }

    public E removeLast() {
        return remove(size - 1);
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
//        Node cur = dummyHead.next;
//        while (cur != null){
//            res.append(cur + "->");
//            cur = cur.next;
//        }
        for (Node cur = dummyHead.next; cur != null; cur = cur.next)
            res.append(cur + "->");
        res.append("Null");
        return res.toString();
    }
}
